Add CLAUDE.md: local builds and RTSP checks in pinned Docker images

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Local builds and checks run in Docker
+
+Nothing is installed on the host: no toolchains, JDK, Android SDK or ffmpeg. Every image tag
+below is pinned. The direct `docker run` commands also pass `--pull never`, so a check never
+pulls a different version than the last run. `android/build.sh` calls `docker run` without
+that flag; with a local image Docker does not pull anyway.
+
+Toolchains are cached on the host in `~/.cache/compot/toolchains` and mounted read-only at
+`/opt/tc`. The image sets `TOOLCHAIN_CACHE=/opt/tc`, and `ci/build-musl.sh` reads its cache
+from there instead of its default `~/.cache/vtesse-toolchains`. A read-only mount cannot
+take a new toolchain: to download one for the first time, mount it read-write once.
+
+```sh
+# native build and unit tests (what the CI "Unit tests" job runs)
+docker run --rm --pull never -u "$(id -u):$(id -g)" -v "$PWD:/src" -w /src compot/cross:2026.10 \
+  sh -c 'make -j"$(nproc)" BUILD=build-dev && make -j"$(nproc)" BUILD=build-dev test'
+
+# static musl binaries for a camera or router target, unit tests under qemu-user
+docker run --rm --pull never -u "$(id -u):$(id -g)" -v "$PWD:/src" \
+  -v ~/.cache/compot/toolchains:/opt/tc:ro -w /src compot/cross:2026.10 \
+  sh -c 'QEMU=1 ci/build-musl.sh arm32 dist'
+
+# Android APKs
+VTESSE_ANDROID_IMAGE=compot/android:2026.10 android/build.sh assembleDebug
+```
+
+`build-*/` and `dist/` are gitignored.
+
+## Checking an RTSP stream
+
+Use the pinned `jrottenberg/ffmpeg:6.1-alpine`. It includes `ffprobe` and the RTSP demuxer.
+Never run `jrottenberg/ffmpeg` without a tag: Docker pulls `latest`, about 300 MB, which
+nothing else uses.
+
+```sh
+docker run --rm --pull never --network host --entrypoint ffprobe jrottenberg/ffmpeg:6.1-alpine \
+  -hide_banner -rtsp_transport tcp -i rtsp://<user>:<pass>@<host>:<port>/<path>
+docker run --rm --pull never --network host jrottenberg/ffmpeg:6.1-alpine \
+  -hide_banner -rtsp_transport tcp -i rtsp://... -t 5 -f null -
+```
+
+## Rules
+
+- Local builds are for development only. Releases, pre-releases and builds for testers come
+  from CI.
+- `bench/bench.sh` builds and runs its own `vtesse-bench` image. The `vtesse-bench-server`
+  container may be in use by another session: check `docker ps` before running `bench.sh` or
+  removing anything.